Biff, Chip and Kipper books are a rite of passage for most children. Parents and kids plough through the whole set, getting to know the three siblings, their mum and dad and of course Floppy the dog.

It all seems very innocent - but all is perhaps not quite as it seems.

The Oxford Reading Tree series has helped primary schoolchildren learn to read for more than 30 years. The books are used in 80 per cent of British schools and there are more than 800 titles in the series.